In general, a silent chain is composed of a plurality of guide row plates, sometimes referred to as “middle” plates and link row plates, sometimes referred to as “inner” plates. In most silent chains, each such plate has a pair of pin holes and a pair of teeth. The plates are arrayed respectively in guide rows, and non-guide rows also referred to as “link rows.” The plates of the guide rows and the link rows are interleaved in alternating relationship in such a way that the pin holes of the plates of each guide row overlap pin holes of the plates of two adjacent link rows. Guide plates, each having a pair of pin holes, are disposed at outermost sides of the guide rows. The rows of plates are connected by connecting pins that extend through the overlapping pin holes. The connecting pins can be in the form of sets of rocker pins, each set consisting of two elongated rocker pins having convex rocker surfaces facing and in engagement with each other, with one of the two pins in each set being secured at both ends to guide plates on opposite sides of the chain.
The silent chain is engaged with a sprocket in various ways depending on the shape of its teeth. There are two main types of silent chain. One is the so-called “inner flank engaging, outer flank seating” type silent chain, in which the inner flanks of the teeth of the chain contact, and slide on, the sprocket teeth as the chain approaches the sprocket. The other main type of silent chain is the so-called “outer flank engaging, outer flank seating” type silent chain, in which outer flanks of the teeth of the chain contact, and slide on, the sprocket teeth as the chain approaches the sprocket. Examples of both types of silent chain are described in laid-open Japanese Patent Applications 2008-138789 and 2000-329199, and U.S. Pat. No. 6,334,828, dated Jan. 1, 2002.
When the teeth of the respective plates of an “inner flank engaging, outer flank seating” type silent chain engage sprocket teeth, an inner flank of a tooth at the front of a plate (with reference to the direction of advance of the chain) first engages a forward flank of a sprocket tooth. Then, as the chain moves onto the sprocket, the engagement point moves along the front flank of the sprocket tooth toward the bottom of the sprocket tooth. As the chain continues to move onto the sprocket, engagement shifts from engagement of the inner flank of the front tooth with the forward flank of a sprocket tooth to engagement of the outer flanks of both teeth with the rear and front flanks respectively of sprocket teeth preceding and following the sprocket tooth first engaged. Consequently, when the plate is fully seated on the sprocket, only the outer flanks contact the sprocket teeth. In this case, an inner flank and both outer flanks of the plate contact the sprocket tooth in the process of engagement.
In the case of the “outer flank engaging, outer flank seating type” silent chain, an outer flank of a rear tooth of a plate first contacts a forward flank of a sprocket tooth, and the engagement point moves along the same flank until the plate is fully seated. On the way, the outer flank of the front tooth of the plate also engages a sprocket tooth, two teeth ahead of the tooth first engaged, and both outer flanks of the plate become seated when engagement is complete. In this case, only the outer flanks of the teeth of the plate contact sprocket teeth, and the inner flanks do not contact the sprocket teeth at all.
FIG. 10 shows a plate of a typical prior art silent chain. The plate can be either a link row plate 510 or a guide row plate 520. If the plate is a link row plate 510, it has a pair of pin holes 511 and a pair of teeth 512, and is arranged with other similar plates in a link row JL as shown in FIG. 11. If the plate is a guide row plate 520, it has a pair of pin holes 521 and a pair of teeth 522, and is arranged along with other similar plates in a guide row GL. Guide plates, each having a pair of pin holes 531, are disposed along the sides of the chain at the outermost ends of the guide rows GL. First and second rocker pins 541 and 542, having convex circular arc-shaped rocker surfaces, are inserted into the pin holes 511 and 521 of the plates 510 and 520 with their convex rocker surfaces facing each other.
The link row plates and guide row plates 510 and 520, respectively in the link rows JL and in the guide rows GL, are arranged in alternate, interleaved, relationship as shown in FIG. 11. The first rocker pins 541 are fixed in the pin holes 531 of the guide plates 530. The second rocker pins 542 are shorter than the pins 541, and their length is slightly less than the distance between the guide plates, and they are maintained in place by engagement of their ends with the guide plates.
As shown in FIG. 10, the pin holes 511 and 521 of the link row plates and guide row plates have convex portions 513 and 523 that engage correspondingly shaped grooves formed on the rocker pins 541 and 542 to prevent rotation of the each pin relative to the plate having the convex portion with which its groove is engaged. The portions of the pin holes 511 and 521 other than the convex parts 513 and 523 are not exactly circular. The pin holes are slightly elongated by virtue of side portions 514, or 524, that give the pin holes a slightly oval shape. The purpose of the oval shape is to alleviate the force that tends to widen the pin holes in the longitudinal direction of the chain. This force is caused when the contact points of the convex arcuate surfaces of the rocker pins 541 and 542 move, and the distance between the portions of the two rocker pins furthest from the contact points increases slightly, as described in Japanese laid-open patent application Hei. 10-122309.
In both types of silent chain, the link row plates 510 move longitudinally relative to the adjoining guide row plates 520 during the time from the beginning of engagement of the link row plates to seating on the sprocket. Similarly, the guide row plates 520 move longitudinally relative to the adjoining link row plates as the guide row plates become seated. These relative movements enable the teeth 512 and 522 to engage smoothly with the sprocket teeth. The extent of relative movement is generally greater in the case of an inner flank engaging, outer flank seating, type chain.
As shown in FIGS. 10 and 11, the rocker pins 541 and 542 are inserted into the pin holes 511 and 521 with only a slight gap resulting from the oval shape due to the slight elongation corresponding to side portions 514 and 524 of the pin holes. No consideration has been given in the prior art to the problem that a large impact sound is generated when the relative movement at the beginning of engagement due to the shape of the teeth of the chain occurs as a result of a large reactive force from adjoining plates which have already been seated.
Another problem with the conventional silent is that the life of the chain is shortened by the repetitive load applied to the pins and to the pin holes by the reactive force at the beginning of engagement.